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Time for the third instalment of Niall's video diary that he recorded in the field in Colorado last summer. This time he almost talks about science. Time to calibrate the eddy flux correlation time lag! It goes like this...By knowing the direction of the wind and the concentration of something, we can work out the "flux", that is, how much of the something is being emitted from the forest into the atmosphere (if the something is greater when the wind is blowing up the way) or how much of something is sinking from the atmosphere into the forest (if something is greater when the wind is, you guessed it, blowing down). The problem is, for the calculations to work we have to do this superfast - about 20 times every second.We used a "sonic anemometer" which measures wind direction by detecting changes is the speed of sound between some little nodules: if there is a gust of wind, then the sound takes longer or shorter to travel over the same distance. At the same time, we are sampling air from right next to the sonic anemometer by sucking it down a tube. This goes to an instrument in the box at the bottom which makes measurements of the particles that are suspended in the air. The problem is that, because the air takes a certain amount of time to travel down the pipe before it is measured, there is a "lag time" between the wind measurement and the particle measurements. If we know this lag time then we can just shift the measurements so they match.That's where the balloons, cigarettes and a sharp knife come in. Cigarette smoke is made up of lots of particles. The idea is that we fill the balloon with smoke and pop it right next to the sonic anemometer and the particle tube. The sonic detects the pressure wave from the balloon pop (effectively a very small fast gust of wind) and the instantaneous release of particles is measured (after they've travelled down the tube) by the instrument at the bottom. We just need to match up the pop on the sonic with the particles detected after the tube and, hey presto!, there's the lag time.

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Here's the second instalment of the Barometer Vodcast where we show you some of the experiments we were performing in the field in Colorado this summer. Tune in next time to see what I needed balloons, cigarettes and Stanley knife for.

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Here is the inaugural Barometer Podcast Vodcast! Join Niall on his adventures making field measurements in Colorado. This episode just introduces the location with some future episodes looking at the kinds of measurements we make and what day to day life is like on fieldwork. The project is looking at the reason its been raining less in central USA by trying to find out how natural emissions from forests affect rainfall.Remember to get your questions in for the live event next month as part of the Manchester Science Festival!

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